This invention relates to a system for controlling the idling speed of an internal combustion engine.
In general, various control variables such as the amount of intake air and ignition timing of an engine in the idling state are decided in advance to conform to the state of a load such as an air conditioner. However, there are cases where the idling speed originally determined cannot be obtained, even if idling is regulated in accordance with these control variables, owing to a variance in throttle opening or in increase in resistance that accompanies aging of the engine.
An example of means for dealing with this problem is disclosed in e.g. the specification of Japanese Patent Application Laid-Open No. 56-121843 (the corresponding U.S. patent of which is U.S. Pat. No. 4,328,775). Specifically, when an engine is running in the idling state, an error is sensed between actual engine speed and a desired idling speed, and the ignition timing of the engine is corrected in dependence upon the error, whereby the actual engine speed is subjected to feedback control so as to attain a target value. In a system of this kind, the difference between actual rotational speed and the target rotational speed is calculated by processing and the engine control variables are decided based on the results of this processing. Consequently, the control system is complicated. In adition, owing to the time needed to execute the processing, control timing is late and it is difficult to control engine rpm to the target rpm with a quick response.
In other words, even if engine speed is dropping when the error between the actual speed and the target speed is sensed, in many instances engine speed is actually on the increase at the moment the aforementioned processing ends. If control is effected in such case so as to raise the engine speed, the end result will be to promote a fluctuation in engine speed rather than the stabilization thereof.